1. Technical Field
The invention disclosed and claimed herein generally pertains to a system and method for monitoring thermal conditions in a processor chip or other integrated circuit (IC), in order to detect unacceptable temperature levels. More particularly, the invention pertains to a system of the above type wherein thermal sensors placed in a chip or IC each includes an oscillator for generating signal pulses at a frequency that varies as a function of the adjacent temperature. Even more particularly, the invention pertains to a system of the above type wherein the oscillator pulses are counted during sample acquisition periods of fixed or unvarying time duration.
2. Description of Related Art
Previously, thermal sensors for use in processors or other integrated circuits, such as IBM power processors, have used the functional clock of the processor to sample an asynchronous thermally sensitive device such as a ring oscillator (PSRO). More particularly, it was realized that the frequency produced by a PSRO varies as a function of the adjacent or proximate temperature. Thus, by sampling the oscillation frequency during successive time intervals, the adjacent temperature may be measured. However, it is clear that the above prior art method makes the temperature measurement dependent on the processor frequency, since the measurement is a function of the time intervals used to sample the PSRO oscillations. As indicated above, these time intervals are determined by the processor clock frequency.
Processors containing sensors of the type described above for thermal monitoring may be used in systems where the processor clock frequency, or reference frequency, requires spread spectrum, or constant dynamic frequency variation. Spread spectrum may be used in order to reduce electromagnetic interference (EMI). In this technique the reference frequency used to determine the thermal sampling intervals could, for example, nominally be 3.0 GHz. However, with spread spectrum the reference frequency is in fact deliberately varied, such as between 2.99 GHz and 3.01 GHz, in order to avoid excessive noise generation. Moreover, frequency slewing could be applied to the reference frequency, that is, temporary reduction of frequency to reduce power.
It has been found that the above variations in processor reference frequency can introduce significant error into the measurement of temperature. This is because apparent variations in temperature reading, that are in fact due to deliberate changes in reference frequency, are indistinguishable from the actual variations in temperature that are intended to be monitored. Thus, a user of the above prior art approach for temperature sensing could not be sure that the true temperature was being determined. This uncertainty has made it necessary to heavily guardband the “acceptable” temperature range of the hardware or software that is relying on the temperature sensor readings for its proper operation.